THINKING OF BUYING A TOXICITY MONITOR FOR RAPID WASTEWATER SCREENING?

With the cost of toxicity testing with surrogate aquatic species forever rising, many pretreatment facilities and POTWs are starting to look for an affordable alternative. Coupled with the delay of days to weeks before data is available, many plant managers, treatment plant operators, and regulatory agencies are placing increased emphasis on monitoring techniques that are on-site. Rapid monitoring techniques are valuable for both industries and municipalities because they make data available in a preventive, rather than historical, time frame.

There are currently several technologies available to provide on-site toxicity monitoring of wastewater. One method uses marine bacteria which emit light which changes as their metabolism is impacted by the waste. Another method measures the amount of enzyme present in the wastewater to estimate the health of the system. A third system uses respirometry to measure the oxygen uptake rate of the bacteria in the wastewater. A disadvantage of the above systems is that they are batch systems which do not give continuous data and can take from an hour to several hours to yield information.

A truly on-line toxicity monitor is available which uses a biological filter seeded with the POTW's bacterial population from the activated sludge. Aerobic microorganisms consume oxygen as they digest biodegradable material. If this activity is normal, dissolved oxygen levels decrease. Inhibitory or toxic materials impair biomass growth and respiration, and little or no dissolved oxygen is consumed. The quantity of oxygen consumed is, therefore, an indication that the waste is treatable by or inhibitory to the biomass of the plant's activated sludge process.

Where an industrial facility is the source of toxic or inhibitory wastes, an on-line monitor provides real time response, recording and alarm capability to allow maximum time for remedial action to be taken. Ideally, this monitor should be installed on the effluent of the pretreatment facility, because this allows time for remedial action by the industry, and a period of warning to the POTW of a potential problem.

To fill the requirements of a true on-line toxicity monitor the instrument must:

1. Show the effect of influent on the plant's own biomass.
2. Be supplied with a continuous, representative sample.
3. Have a rapid response time.
4. Provide alarms when inhibitory or toxic conditions begin.
5. Require minimum operator attention.

Because of the critical nature of the information the monitor is providing, it must also be able to:

1. Check its own operation and calibration with suitable alarms in the event of a problem.
2. Provide a permanent record of "nontoxic" as well as problem conditions.
3. Provide data in a form that can be easily compared and used in reports.

Consider the following check list before you purchase to assure that you get the monitor you need and are not paying for features you don't need or want.

What application needs monitoring?
( ) Toxicity or inhibition of influent water
( ) Toxicity or inhibition of the effluent
( ) Toxicity or inhibition of individual waste or process streams
( ) Toxicity or inhibition testing for final discharge water
( ) Treatability of Industrial outfall as part of an industrial pretreatment program

How often will the monitor be used?
( ) Continuous, on-line, ( ) Hourly, ( ) Daily, ( ) Once a week,
( ) Once a month, ( ) As needed, ( ) When storm or other event occurs
( ) Other ____________

Where will the monitor(s) be located?
( ) Indoor location, ( ) Outdoor location, ( ) Portable

The Basic monitor:
==================== Important ===== Not critical=== Not applicable
Plumbed in place:=========( ) ============( )============( )
Portable:=================( ) ============( )============( )
Size / Weight:============( ) ============( )============( )
Wall mount:===============( ) ============( )============( )
Lockable:=================( ) ============( )============( )
Self calibrate:===========( ) ============( )============( )
Unattended operation:=====( ) ============( )============( )

Sample Inlet:
Lift from liquid to monitor ( ft.) Length of horizontal run ( ft.)
Solids handling capacity: ( ) dissolved ( ) light solids ( ) coarse solids

Power :
( ) AC (line power only)
( ) DC (Battery Only)

Programming:
What type of endpoints are required?
( ) Set inhibition alarm point
( ) Set toxicity alarm point
( ) Provide remote alarm contacts

Method of programming:
( ) Switch select (all functions visible to operator as set)
( ) Keypad Programmable (operator input and interrogation to check selected program)
( ) A combination of the above.

Special Functions:
( ) Data download to a standard PC format
( ) Uses the POTW activated sludge instead of a surrogate organism
( ) Can use commercially available screening cultures or bacterial

With this checklist in mind, read manufacturers literature and check the features you need. Feel free to contact the manufacturers or their representatives if you have any questions or specific needs. On-line monitors are valuable tools that are cost effective if they are simple to use and maintain and are reliable in the timely and detailed data to reduce inhibitory discharges to biological treatment processes. Receiving waters can be protected from discharges of untreated wastes from plants which have experienced process upsets and resulting toxic material pass through.

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